The flexible direct current (DC) transmission is at a new transmission method, which adopts direct current for transmission and uses the turn-off voltage source converters and advanced modulation technology to carry out DC transmission. It has some advantages, like supplying power for isolated grid, without commutation failure, no need for reactive power compensation, being able to control flexibly active and reactive power output etc, and it is also a beneficial supplement for the alternating current (AC) network, to achieve the power delivery and to improve the existing AC transmission capability and stability, so as to make the existing AC transmission system more effective. The modular multilevel converter is composed of multiple sub modules in series, and the sub module is composed of a bypass switch, a thyristor, an IGBT, a diode and a capacitor.
The first state MMC-HVDC project will be in operation in Shanghai Nanhui wind power plant. However, the converter valve level number is less than 15 of the present physical simulation prototype which is designed for the MMC-HVDC engineering, it has large volume and high voltage, and RTDS can only realize off-line test.
In these figures:
L1 is an adjustable reactor; L2 is the converter; QS1 is the vacuum switch I, QSII is the vacuum switch II, QSIII is the vacuum switch III; QF1 is the contactor I, QF6 is the contactor II; QF2 is the contactor III, QF3 is a contactor; TAx is the Holzer current sensor; TVx is the Holzer voltage sensor; Fx is a fuse; and R1 is resistors.